1. Field of the Invention
This invention relates to compounds for the inhibition of histone deacetylase (HDAC). More particularly, the invention relates to compounds and methods for inhibiting HDAC enzymatic activity.
2. Description of Related Art
Histone deacetylases are involved in the epigenetic regulation of gene expression through chromatin remodeling. In eukaryotic cells, nuclear DNA associates with histones to form a compact complex called chromatin. The histones constitute a family of basic proteins which are generally highly conserved across eukaryotic species. The core histones, termed H2A, H2B, H3, and H4, associate to form a protein core. DNA winds around this protein core, with the basic amino acids of the histones interacting with the negatively charged phosphate groups of the DNA. Approximately 146 base pairs of DNA wrap around a histone core to make up a nucleosome particle, the repeating structural motif of chromatin.
Csordas, Biochem. J., 286: 23-38 (1990) teaches that histones are subject to posttranslational acetylation of the N-terminal lysine residues, a reaction that is catalyzed by histone acetyl transferase (HAT1). Acetylation neutralizes the positive charge of the lysine side chain, and is thought to impact chromatin structure. Indeed, Taunton et al., Science, 272: 408-411 (1996), teaches that access of transcription factors to chromatin templates is enhanced by histone hyperacetylation. Taunton et al. further teaches that an enrichment in underacetylated histone H4 has been found in transcriptionally silent regions of the genome.
Histone acetylation is a reversible modification, with deacetylation being catalyzed by a family of enzymes termed histone deacetylases (HDACs). The molecular cloning of gene sequences encoding proteins with HDAC activity has established the existence of a set of discrete HDAC enzyme isoforms. Histone deacetylases play an important role in gene regulation in mammalian cells. Gray and Ekstrom, Expr. Cell. Res. 262: 75-83 (2001); Zhou et al., Proc. Natl. Acad. Sci. USA 98: 10572-10577 (2001); Kao et al. J. Biol. Chem. 277: 187-193 (2002) and Gao et al. J. Biol. Chem. 277: 25748-25755 (2002) teach that there are 11 members of the histone deacetylase (HDAC) family.
Class I histone deacetylases include HDAC1, HDAC2, HDAC3 and HDAC8. The Class I enzymes are expressed in a wide variety of tissues and are reported to be localized in the nucleus. Class II histone deacetylases include HDAC4, HDAC5, HDAC6, HDAC7, HDAC9 and HDAC10. The Class II enzymes have been described as limited in tissue distribution and they can shuttle between the nucleus and the cytoplasm. The Class II enzymes are further divided into Class IIa (HDAC4, HDAC5, HDAC7 and HDAC9) and Class IIb (HDAC6 and HDAC10). Recent classifications place HDAC11 in a class of its own.
Studies utilizing known HDAC inhibitors have established a link between acetylation and gene expression. Numerous studies have examined the relationship between HDAC and gene expression. Taunton et al., Science 272:408-411 (1996), discloses a human HDAC that is related to a yeast transcriptional regulator. Cress et al., J. Cell. Phys. 184:1-16 (2000), discloses that, in the context of human cancer, the role of HDAC is as a corepressor of transcription. Ng et al., TIBS 25: March (2000), discloses HDAC as a pervasive feature of transcriptional repressor systems. Magnaghi-Jaulin et al., Prog. Cell Cycle Res. 4:41-47 (2000), discloses HDAC as a transcriptional co-regulator important for cell cycle progression.
Richon et al., Proc. Natl. Acad. Sci. USA, 95: 3003-3007 (1998), discloses that HDAC activity is inhibited by trichostatin A (TSA), a natural product isolated from Streptomyces hygroscopicus, which has been shown to inhibit histone deacetylase activity and arrest cell cycle progression in cells in the G1 and G2 phases (Yoshida et al., J. Biol. Chem. 265: 17174-17179, 1990; Yoshida et al., Exp. Cell Res. 177: 122-131, 1988), and by a synthetic compound, suberoylanilide hydroxamic acid (SAHA). Yoshida and Beppu, Exper. Cell Res., 177: 122-131 (1988), teaches that TSA causes arrest of rat fibroblasts at the G1 and G2 phases of the cell cycle, implicating HDAC in cell cycle regulation. Indeed, Finnin et al., Nature, 401: 188-193 (1999), teaches that TSA and SAHA inhibit cell growth, induce terminal differentiation, and prevent the formation of tumors in mice. Suzuki et al., U.S. Pat. No. 6,174,905, EP 0847992 and JP 258863/96, disclose benzamide derivatives that induce cell differentiation and inhibit HDAC. Delorme et al., WO 01/38322 and WO 2001/070675, disclose additional compounds that serve as HDAC inhibitors. Other inhibitors of histone deacetylase activity, including trapoxin, depudecin, FR901228 (Fujisawa Pharmaceuticals), and butyrate, have been found to similarly inhibit cell cycle progression in cells (Taunton et al., Science 272: 408-411, 1996; Kijima et al., J. Biol. Chem. 268(30):22429-22435, 1993; Kwon et al., Proc. Natl. Acad. Sci. USA 95(7):3356-61, 1998).
These and other findings suggest that inhibition of HDAC activity represents a novel approach for intervening in cell cycle regulation and that HDAC inhibitors have great therapeutic potential in the study and/or treatment of diseases or conditions ameliorated by modulating HDAC activity. Inhibitors which are not selective for specific HDAC isotype(s) may have undesirable side effects than an HDAC inhibitor that is better able to target specific HDAC isoforms, and may thus be less desirable as an inhibitor or therapeutic agent. Identification of inhibitors selective for specific HDAC isotype(s) will yield novel strategies for understanding the role of histone deacetylases and treating diseases ameliorated by modulating by HDAC activity.